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DAC161P997: how will the 47uF bulk capacitances affect the loop current

Part Number: DAC161P997

Dears,

I use DAC161P997 to build 4-20mA. And I parallel one 47uF bulk capacitance to the loop receiver resistance. And there is a significant ripple measured as below. Vpp=200mV 

Do you know about how to analyze the effects for this 47uF cap in the application?Thanks.

  • Tess,

    Out of curiosity, what is the frequency of this ripple? It does look like some sort of oscillation.

    I'm not sure I have a good way to analyze the addition of the 47uF capacitance. This seems like a very large capacitance, and it seems that it would make this loop unstable. The datasheet does discuss capacitances in the loop and how they make the loop unstable. This is shown in figure 24 of the datasheet.

    It seems to me that the capacitance in your circuit is similar to capacitance on LOOP- in the figure shown as Cx4. There is a brief discussion on the stability in section 8.1.1.2.4 on page 25 of the datasheet. It does say to treat Cx4 similar to Cx1 and the capacitance should not exceed 10nF (without any series resistance).

    One other important thing to note is that if you have the oscilloscope connected to multiple measurement points, you may inadvertently connect the device ground (COMA) to the loop ground (the ground of the load resistor). This could cause a problem with device controlling the loop.

    Regardless, I think the 47uF capacitance must be lowered because of the stability of the loop.


    Joseph Wu